Polyacrylonitrile-based carbon fiber bundles, with two sides fixed, were heated in a graphitization furnace with temperature profiles similar to those used in the preparation of carbon/carbon composites. The microstructure of the microvoids formed was characterized by small-angle X-ray scattering and high resolution transmission electron microscopy (HR-TEM). Results showed that the average radius of gyration of the microvoids decreases, and their length, width, cross-sectional area and volume fraction decrease followed by a slight increase with increasing temperature, with the minima found at around 2300°C. The HR-TEM images showed that the microvoids were formed by stacking defects of the carbon layers. The ordering of the amorphous carbon layers accounted for the initial decrease of size and volume fraction of the voids with increasing temperature, while the growth and ordering of the layers at high temperatures gave rise to increases in their size and volume fraction.